The present invention relates to high-performance oxide superconductors and oxide superconductor composites. The present invention further relates to a method for healing defects introduced into the oxide superconductor phase during processing thereby improving superconducting properties. The present invention also relates to the processing of high performance bismuth-strontium-calcium-copper oxide superconductors and oxide superconductor composites and a method for improving the critical transition temperature (Tc and critical current density (Jc) of these oxide superconductors.
Oxide superconductors of the rare earth-barium-copper-oxide family (YBCO), bismuth(lead)-strontium-calcium-copper-oxide family ((Bi,Pb)SCCO) and thallium-barium-calcium-copper-oxide family (TBCCO) form plate-like and highly anisotropic superconducting oxide grains. Because of their plate-like morphology, the oxide grains can be oriented by mechanical strain. Mechanical deformation has been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. The degree of alignment of the oxide superconductor grains is a major factor in the high critical current densities (Jc) obtained in articles prepared from these materials.
Known processing methods for obtaining textured oxide superconductor composite articles include an iterative process of alternating heating and deformation steps. The heat treatment is used to promote reaction-induced texture of the oxide superconductor in which the anisotropic growth of the superconducting grains is enhanced. Each deformation provides an incremental improvement in the orientation of the oxide grains. Additional heat treatment intermediate with or subsequent to deformation is also required to form the correct oxide superconductor phase, promote good grain interconnectivity and achieve proper oxygenation.
Processing long lengths of oxide superconductor is particularly difficult because deformation leads to microcracking and other defects which may not be healed in the subsequent heat treatment. Cracks that occur perpendicular to the direction of current flow limit the performance of the superconductor. In order to optimize the current carrying capability of the oxide superconductor, it is necessary to heal any microcracks that are formed during processing of the oxide superconductor or superconducting composite.
Liquid phases in co-existence with solid oxide phases have been used in processing of oxide superconductors. One type of partial melting, known as peritectic decomposition, takes advantage of liquid phases which form at peritectic points of the phase diagram containing the oxide superconductor. During peritectic decomposition, the oxide superconductor remains a solid until the peritectic temperature is reached, at which point the oxide superconductor decomposes into a liquid phase and a new solid phase. The peritectic decompositions of Bi2Sr2CaCu2O8+x, (BSCCO-2212, where 0xe2x89xa6xxe2x89xa61.5), into an alkaline earth oxide and a liquid phase and of YBa2Cu3O7xe2x88x92xcex4, (YBCO-123, where 0xe2x89xa6xcex4xe2x89xa61.0) into Y2BaCuO5 and a liquid phase are well known.
Peritectic decomposition of an oxide superconductor and the reformation of the oxide superconductor from the liquid + solid phase is the basis for melt textured growth of YBCO-123 and BSCCO-2212. For example, Kase et al. in IEEE Trans. Mag. 27(2), 1254 (MarCu 1991) report obtaining highly textured BSCCO-2212 by slowly cooling through the peritectic point. This process necessarily involves total decomposition of the desired 2212 phase into an alkaline earth oxide and a liquid phase.
It is also recognized that an oxide superconductor itself can co-exist with a liquid phase under suitable processing conditions. This may occur by solid solution melting, eutectic melting or by formation of non-eqilibrium liquids.
Solid solution melting may arise in a phase system, in which the oxide superconductor is a solid solution. As the temperature (or some other controlling parameter) of the system increases (or decreases), the oxide superconductor phase changes from a solid oxide phase to a liquid plus oxide superconductor partial melt (this happens at the solidus). A further increase in temperature (or some other controlling parameter) affords the complete melting of the oxide superconductor (this happens at the liquidus).
A phase diagram containing a eutectic point may provide an oxide superconductor partial melt, known as eutectic melting, when the overall composition is chosen so as to be slightly off stoichiometry. As the temperature (or some other controlling parameter) of the system increases (or decreases), the mixed phase of oxide superconductor-plus-non-superconducting oxide (solid1+solid2) changes to a liquid-plus-oxide superconductor partial melt (solid1+liquid).
Non-equilibrium liquids may also promote partial melting in oxide superconductor systems. A non-equilibrium liquid is established through the relatively rapid heating of a mixture of oxides to a temperature above the eutectic melting point of local stoichiometries present in the heterogeneous mixture of phases. As the oxides form the desired oxide superconductor, the solid and liquid phases can co-exist, if only temporarily.
Partial melting of (Bi,Pb)2Sr2Ca2Cu3O10+x ((Bi,Pb)SCCO-2223, where 0xe2x89xa6xxe2x89xa61.5) and (Bi)2Sr2Ca1, Cu2O10+x ((Bi)SCCO-2223, where 0xe2x89xa6xxe2x89xa61.5), 25 BSCCO-2223, at temperatures above 870xc2x0 C. in air has been reported; see, for example, Kobayashi et al. Jap. J. Appl. Phys. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055 (Nov. 1988), Luo et al. Appl. Super. 1, 101-107, (1993), Aota et al. Jap. J. Appl. Phys. 28, L2196-L2199 (1989) and Luo er al. J. Appl. Phys. 72, 2385-2389 (1992). The exact mechanism of partial melting of BSCCO-2223 has not been definitively established.
Guo et al. in Appl. Supercond. 1(1/2), 25 (January 1993) have described a phase formation-decomposition-reformation (PFDR) process, in which a pressed sample of (Bi,Pb)SCCO-2223 is heated above its liquidus to decompose the 2223 phase, followed by a heat treatment at a temperature below the solidus. The sample was subsequently pressed again and reannealed. The final anneal of the PFDR process includes a standard single step heat treatment in which there is no melting.
The xe2x80x9chigh Tcxe2x80x9d oxide superconductor Bi2xe2x88x92yPbySr2Ca2Cuxe2x88x9210+x, where 0xe2x89xa6xxe2x89xa61.5 and 0xe2x89xa6yxe2x89xa60.6 (BSCCO-2223 and (Bi, Pb)SCCO-2223, hereinafter referred to as xe2x80x9cBSCCO-2223xe2x80x9d to indicate both lead-doped and undoped compositions), is particularly desirable because of its high critical transition temperature (Tcxcx9c110 K) and high critical current (Ic, Jc). The superconducting art constantly seeks to improve electrical properties, such as, critical current density and critical transition temperature.
Partial melting in the processing of oxide superconductors has been used either to increase the yield of the BSCCO-2223 phase or to improve the contiguity and texturing of the oxide superconductor grains. The issue of healing defects, such as microcracks, which develop during processing of the oxide superconductor, has not been addressed. Further, the prior art has not addressed the possibility of using a two-step process where the oxide superconductor is stable in both steps for the healing of cracks and defects.
Wang et al. (xe2x80x9cAdvances in Superconductivityxe2x80x9d, Springer-Verlag, New York, Editors: Y. Bando and H. Yamauchi, pp. 291-294 (1993)) report an increase in Tc by carrying out a post-anneal step at 790xc2x0 C. at reduced total pressures. Wang et al. observed Tc by DC magnetization of 115 K and Tc,zero of 111 K by resistivity measurement. The technique used by Wang et al. (vacuum encapsulation at 104 Torr of oxide superconductor pellets, followed by annealing at 790xc2x0 C.) does not permit determination of the oxygen pressure of the system. The encapsulated pellets reach an equilibrium oxygen pressure within the capsule by releasing oxygen. The pellet volume/capsule volume plays an important role in determining the final equilibrium oxygen pressure.
Critical transition temperatures (determined by magnetization) as high as 117 K have been reported for multiphase materials containing BSCCO-2223. Fisher et al. (Physica C 160, 466 (1990)) reported a Tc of 115 K (determined by magnetization) with the substitution of lead and antimony in the BSCCO-2223 system. A non-reproducible Tc as high as 130 K was reported by Hongbo et al. (Solid State Comm. 69, 867 (1989)).
While reports of high transition temperatures by magnetization studies are of interest, they can sometimes be misleading. The transition curves obtained by magnetization are xe2x80x9csoftxe2x80x9d, making extrapolation to zero resistivity highly subjective. Further, other effects, such as semiconductor to metallic transitions, can mimic critical temperature transition behavior. It is therefore desirable to rely on bulk resistivity measurements for determining temperature at zero resistivity (Tc, zero).
Idemoto et al. (Physica C 181, 171-178 (1991)) has investigated the oxygen content and copper and bismuth valances of BSCCO-2223 under a range of conditions, including temperatures in the range of 500xc2x0 C. to 850xc2x0 C. and oxygen pressures in the range of 0.005 to 0.20 atm. The samples were observed by means of a microbalance under changing temperatures conditions at constant oxygen pressures. Because the samples do not reach equilibrium during the observation period, it is difficult to determine the exact processing conditions experienced by the samples. No investigation of the effect of reported conditions on electrical properties is reported.
None of the previous research has indicated the desirability of post-annealing the BSCCO-2223 phase at low temperatures and oxygen pressures to enhance the electric transport properties of the oxide superconductor, namely critical current.
It is the object of the present invention to provide a method for improving superconducting performance of oxide superconductors and superconducting composites by healing cracks and defects formed during processing of oxide superconductors and superconducting composites.
It is a further object of the invention to prepare oxide superconducting articles having significantly less cracks and defects than conventionally-processed articles.
A further object of the present invention is to provide a process to increase the critical current density of BSCCO-2223 by a method which also increases its critical transition temperature. It is a further object of the present invention to provide a novel high-Tc BSCCO-2223 composition having a critical transition temperature greater than 111.0 K.
A feature of the invention is a two-step heat treatment after which no further deformation occurs which introduces a small amount of a liquid phase co-existing with the oxide superconductor phase, and then transforms the liquid back into the oxide superconductor phase with no deformation occurring during or after the heat treatment of the invention. A further feature of the present invention is a low temperature, low oxygen pressure anneal of the oxide superconductor.
An advantage of the invention is the production of highly defect-free oxide superconductor and superconducting composites which exhibit superior critical current densities. A further advantage of the invention is a marked improvement in critical transition temperature and critical current density as compared to oxide superconductors and superconducting composites which are not subjected to the method of the invention.
In one aspect of the present invention, an oxide superconductor article containing a desired oxide superconductor phase is exposed to a two-step heat treatment after deformation of the article, which includes (a) heating the article at a temperature sufficient to partially melt the article, such that a liquid phase co-exists with the desired oxide superconductor phase; and (b) cooling the article to a temperature sufficient to transform the liquid phase into the desired oxide superconductor, with no deformation occurring after the heat treatment of the invention.
In another aspect of the invention, an oxide superconductor article containing a desired oxide superconductor phase is exposed to a two-step heat treatment after deformation of the article which includes (a) forming a liquid phase in the oxide superconducting( article, such that the liquid phase co-exists with the desired oxide superconductor solid phase; and then (b) transforming the liquid phase into the desired oxide superconductor, with no deformation occurring after the heat treatment of the invention.
In preferred embodiments, the liquid phase wets surfaces of defects contained within the oxide superconductor article. The defects are healed upon transformation of the liquid to the desired oxide superconductor. The partial melting of step (a) and the transformation of step (b) are effected by selection of appropriate thermodynamic state variables, for example, temperature, PO2, Ptotal and total composition. In principle, deformation may occur during the heat treatment of the present invention up to immediately prior to the completion of step (a), providing that the liquid phase is available for a period of time sufficient to wet defect surfaces.
By xe2x80x9ctwo-step heat treatmentxe2x80x9d or xe2x80x9cheat treatment of the inventionxe2x80x9d, as those terms are used herein, it is meant a heat treatment for forming an oxide superconductor after which no further deformation occurs. However, heat treatments for purposes other than those stated herein, such as, for example, oxygenation of the oxide superconductor, are possible. In all cases, not further deformation occurs.
By xe2x80x9cpartial meltxe2x80x9d, as that term is used herein, it is meant the oxide superconductor article is only partially melted, and that the desired oxide superconductor is present during melting.
By xe2x80x9cdeformationxe2x80x9d as that term is used herein, it is meant a process which causes a change in the cross-sectional shape of the article.
By xe2x80x9coxide superconductor precursorxe2x80x9d, as that term is used herein, it is meant any material that can be converted to an oxide superconductor upon application of a suitable heat treatment. Suitable precursor materials include but are not limited to metal salts, simple metal oxides, complex mixed metal oxides and intermediate oxide superconductors to the desired oxide superconductor.
By xe2x80x9cdesired oxide superconductorxe2x80x9d, as that term is used herein, it is meant the oxide superconductor which it is desired to ultimately prepare. An oxide superconductor is typically the xe2x80x9cdesiredxe2x80x9d oxide superconductor because of superior electrical properties, such as high Tc and/or Jc. The desired oxide superconductor is typically a high Tc member of a particular oxide superconductor family, i.e., BSCCO-2223, YBCO-123, TBCCO-1212 and TBCCO-1223.
By xe2x80x9cintermediate oxide superconductorxe2x80x9d, as that term is used herein, it is meant an oxide superconductor which is capable of being converted into a desired oxide superconductor. However, an intermediate oxide superconductor may have desirable processing properties, which warrants its formation initially before final conversion into the desired oxide superconductor. The formation of an xe2x80x9cintermediate oxide superconductorxe2x80x9d may be desired, particularly during heat treatment/deformation iterations, where the intermediate oxides are more amenable to texturing than the desired oxide superconductor.
In yet another aspect of the present invention, a textured oxide superconductor article is prepared by subjecting an article containing an oxide superconductor precursor to at least one first heat treatment/deformation iteration. The heat treatment is effective to form a desired oxide superconductor. The resultant oxide superconductor phase is textured upon application of the first heat treatment/deformation iteration. The article is then exposed to a two-step heat treatment in which (a) the article is partially melted, such that a liquid phase co-exists with the desired textured oxide superconductor phase; and (b) the liquid phase is transformed into the desired oxide superconductor, with no deformation occurring after the heat treatment of the invention.
In yet another aspect of the present invention, a textured oxide superconductor article is prepared by subjecting an article containing an oxide superconductor precursor to at least one first heat treatment/deformation iteration. The heat treatment is effective to form an intermediate oxide superconductor. The intermediate textured oxide superconductor phase is formed. The article is then subjected to at least one second heat treatment/deformation iteration. The heat treatment is effective to form a desired oxide superconductor. The desired textured oxide superconductor is formed. The article is then exposed to a two-step heat treatment in which (a) the article is partially melted, such that a liquid phase co-exists with the desired textured oxide superconductor phase; and (b) the liquid phase is transformed into the desired oxide superconductor, with no deformation occurring after the heat treatment of the invention.
In preferred embodiments, the intermediate oxide superconductor is BSCCO-2212 or (Bi,Pb)SCCO-2212 because it is readily textured by the heat treatment/deformation iterations. The intermediate oxide superconductor is then converted to a desired oxide superconducting phase, typically BSCCO-2223 or (Bi,Pb)SCCO-2223. The partial melting of step (a) may be carried out at a temperature in the range of 820-835xc2x0 C. at 0.075 atm O2. The transformation of the liquid in step (b) may be carried out at a temperature in the range of 790-820xc2x0 C. at 0.075 atm O2. In other preferred embodiments, the desired oxide superconductor, may be YBCO-123, Y2Ba4Cu7O14xe2x88x92xcex4 (YBCO-247), (Tl,Pb),Ba2Ca1,Cu2O6.0xc2x1y (TBCCO-1212) or (Tl,Pb),Ba2Ca2Cu3O8.0xc2x1y (TBCCO-1223), where 0xe2x89xa6xcex4xe2x89xa61.0 and y ranges up to 0.5. The stated stoichiometries are all approximate and intentional or unintentional variations in composition are contemplated within the scope of the invention.
In other preferred embodiments, the liquid phase is formed in the range of 0.1-30 vol %. In yet other preferred embodiments, the heat treatment of the first and second heat treatment/deformation iterations partially melts the oxide superconductor article.
In yet another aspect of the invention, an oxide superconductor article is exposed to a two-step heat treatment after a deformation step, which includes (a) heating the article at a temperature substantially in the range of 810-860xc2x0 C. for a period of time substantially in the range of 0.1 to 300 hours at a PO2 substantially in the range of 0.001-1.0 atm; and (b) cooling the article to a temperature substantially in the range of 780-845xc2x0 C. for a period of time substantially in the range of 1 to 300 hours at a PO2 substantially in the range of 0.001-1.0 atm, with no deformation occurring after the heat treatment of the present invention.
In yet another aspect of the present invention, an oxide superconductor article containing a desired oxide superconductor phase is exposed to a two-step heat treatment after a deformation step, which includes (a) subjecting the article to an oxygen partial pressure sufficient to partially melt the oxide superconducting article, such that a liquid phase co-exists with the desired oxide superconductor; and (b) raising to an oxygen partial pressure sufficient to transform the liquid phase into the desired oxide superconductor.
Yet another aspect of the present invention provides for a multifilamentary oxide superconductor composite containing a plurality of oxide superconductor filaments contained within a matrix material which has been subjected to the two-step heat treatment of the invention.
In yet another aspect of the invention, a multifilamentary oxide superconductor composite contains a plurality of oxide superconductor filaments contained within a matrix material, the composite having a Jc of at least 14xc3x97103 A/cm2 at 77K, self field, as measured over a length of at least 50 m.
The present invention provides oxide superconductors which exhibit marked improvement in critical current density (Jc) over samples processed in an otherwise similar manner, lacking only the two-step heat treatment of the present invention.
In yet another aspect of the present invention, a BSCCO-2223 oxide superconducting article is prepared by providing an oxide superconductor article including BSCCO-2223 oxide superconductor, and annealing the superconducting article at a temperature selected from the range of about 500xc2x0 C.xe2x89xa6Txe2x89xa6787xc2x0 C. and an annealing atmosphere having an oxygen pressure selected from within the region having a lower bound defined by the equation, PO2(lower)xe2x89xa73.5xc3x971010exp (xe2x88x9232,000/(T+273)) and an upper bound defined by the equation, PO2(upper)xe2x89xa61.1xc3x971010exp(xe2x88x9232,000/(T+273)). The sample is annealed for a time sufficient to provide at least a 10% increase in critical current density as compared to the critical current density of the pre-anneal oxide superconductor article.
In yet another aspect of the invention, a BSCCO-2223 oxide superconducting article is prepared by providing an oxide superconductor article including BSCCO-2223 oxide superconductor, and annealing the superconducting article at a temperature selected from the range of about 500xc2x0 C.xe2x89xa6Txe2x89xa6760xc2x0 C. and an annealing atmosphere having an oxygen pressure selected from within the region having a lower bound defined by the equation, PO2(lower)xe2x89xa78.5xc3x971010exp(xe2x88x9232,000/(T+273)) and an upper bound defined by the equation, PO2(upper)xe2x89xa62.62xc3x971011exp(xe2x88x9232,000/(T+273)). The sample is annealed for a time sufficient to provide at last a 10% increase in critical current density as compared to the critical current density of the pre-anneal oxide superconductor article.
In yet another aspect of the invention, a BSCCO-2223 oxide superconductor article is prepared by exposing the article including at least BSCCO-2223 to a heat treatment after deformation of the article, including (a) heating the article at a temperature substantially in the range of 815-860xc2x0 C. for a period of time substantially in the range of 0.1 to 300 hours at a PO2 substantially in the range of 0.001-1.0 atm; and (b) subjecting the article to a temperature substantially in the range of 790-845xc2x0 C. for a period of time substantially in the range of 1 to 300 hours at a PO2 substantially in the range of 0.01-1.0 atm, with no deformation occurring after the heat treatment. The superconducting article is then annealed at a temperature selected from the range of about 500xc2x0 C.xe2x89xa6Txe2x89xa6787xc2x0 C. and an annealing atmosphere having an oxygen pressure selected from within the region having a lower bound defined by the equation, PO2(lower)xe2x89xa73.5xc3x971010 exp(xe2x88x9232,000/(T+273)) and an upper bound defined by the equation, PO2(upper)xe2x89xa61.1xc3x971012exp(xe2x88x9232,000/(T+273)).
By xe2x80x9canneal of the present inventionxe2x80x9d, it is meant a low pressure, low temperature heat treatment under equilibrium conditions during which no further formation of the desired oxide superconducting phase occurs; however, the internal chemistry of the oxide superconductor (i.e.; oxygen stoichiometry) and grain growth of the existing oxide superconductor phase may be affected.
In preferred embodiments, the annealing atmosphere is substantially at a pressure of one atmosphere and oxygen pressure is obtained by controlling the oxygen concentration in the annealing atmosphere. The annealing atmosphere may additionally contain an inert gas selected from the group consisting of argon, nitrogen and helium. The anneal is carried out at an oxygen pressure substantially in the range of 7.5xc3x9710xe2x88x922 atm to 1xc3x9710xe2x88x928 atm O2. The annealing of the invention is preferably the final annealing to which the superconducting article is subjected.
In other preferred embodiments, the anneal is carried out at a the temperature in the range of 770 to 787xc2x0 C. and an oxygen pressure in the range of 0.017 to 0.085 atm; The anneal is carried out at a temperature in the range of 750 to 770xc2x0 C. and an oxygen pressure in the range of 0.0009 to 0.052 atm; the method of claim 1, 2 or 3, wherein the anneal is carried out at a temperature in the range of 730 to 750xc2x0 C. and an oxygen pressure in the range of 0.005 to 0.029 atm; the anneal is carried out at a temperature in the range of 690 to 730xc2x0 C. and an oxygen pressure in the range of 0.0001-0.015 atm; the anneal is carried out at a temperature in the range of 740 to 760xc2x0 C. and an oxygen pressure in the range of 0.0016 to 0.009 atm; the anneal is carried out at a temperature in the range of 710 to 740xc2x0 C. and an oxygen pressure in the range of 0.0006-0.005 atm; and the anneal is carried out at a temperature in the range of 690 to 710xc2x0 C. and an oxygen pressure in the range of 0.0003-0.002 atm.
In yet other preferred embodiments, sample is annealed at progressively lower temperature and oxygen pressure. This may be accomplished by continuously reducing temperature and/or oxygen pressure or by stepwise reduction of temperature and/or oxygen pressure.
Yet another aspect of the invention includes a superconductor having the formula Bi2xe2x88x92yPbySr2Ca2Cu3O10+x, where 0xe2x89xa6xxe2x89xa61.5 and where 0xe2x89xa6yxe2x89xa60.6, the oxide superconductor characterized by a critical transition temperature of greater than 111.0 K as defined by zero resistance by a four point linear probe method with zero resistance corresponding to a resistivity of less than 108xe2x88x928 xcexa9-cm. An article containing the oxide superconductor may additionally include silver.
Yet another aspect of the invention includes an oxide superconductor article characterized by a critical transition temperature of greater than 111.0 K as defined by zero resistance by a four point linear probe method with zero resistance corresponding to a resistivity of less than 10xe2x88x928 xcexa9-cm and an x-ray diffraction pattern having peaks at 17.4xc2x0, 19.2xc2x0, 20.2xc2x0, 21.8xc2x0, 23.2xc2x0, 23.9xc2x0, 26.2xc2x0, 27.8xc2x0, 29xc2x0, 29.7xc2x0, 31.5xc2x0, 32xc2x0, 33.2xc2x0, 33.7xc2x0, 35xc2x0, 35.6xc2x0, 38xc2x0, 38.8xc2x0, 41.6xc2x0, 43.8xc2x0, 44.4xc2x0, 46.8xc2x0, 47.4xc2x0, 48xc2x0 and 49xc2x0.
Yet another aspect of the present invention provides for a multifilamentary oxide superconductor composite containing a plurality of oxide superconductor filaments contained within a matrix material which has been subjected to an anneal according to the present invention.
The oxide superconductor prepared according to the method of this invention exhibit superior electric transport properties and enhanced Tc.